The present invention relates to a control system for a charging generator, or more in particular to an output voltage control system for a charging generator of an automobile suitably used for charging the battery of the automobile.
Recent years have seen a wide extension of the use of a control system for the automotive charging generator in which the on-off duty factor of the field current is controlled by a chopper. A control system of such a type for a vehicle generator is disclosed in a Japanese patent publication based on an application dated Aug. 28, 1974 filed by Nippon Denso Co., Ltd. and laid open as JP-A-51-25715, a Japanese patent publication based on an application dated July 25, 1978 filed by Nippon Denso Co., Ltd. and laid open as JP-A-55-18839, and a Japanese patent publication based on an application field by Hitachi Ltd. dated July 11, 1980 and laid open as JP-A-57-22338. All of these systems use a fixed frequency method in which the chopping period for control of the field current is kept constant.
The fixed frequency system, in which the pulsation of the field current (ripple) changes depending on the duty factor of the switching operation, has the disadvantage that the performance of the electrical equipment is adversely affected by the pulsation of an output voltage of the generator. Under the worst ripple conditions of the field current, the output voltage of the generator may undergo a vertical fluctuation. In such a case, the headlight or the like flickers undesirably. In order to reduce the ripple, one might consider increasing the switching frequency of the chopper. If the switching frequency is increased into synchronism with the pulsation (ripple) frequency of the output voltage, however, the control operation becomes unstable. A further increase in frequency would cause a radio interference due to high frequency components with noise mixed in radio receiver.
With reference to the specific configuration of a control circuit, the capacity of a capacitor in a time constant circuit of a pulse width modulation (PWM) signal source poses another problem. In other words, an attempt to form a circuit with ICs has encountered a difficulty due to a large capacity of a capacitor.